A variety of polymeric materials are known and have been employed in moisture sensors in past years that have the ability to vary an electrical signal in response to changes in their moisture content. Such materials include cellulose acetate butyrate and certain other cellulose based polymers, poly (styrene-sulfonic acid)--sodium form polymers, poly acrylamide, poly acrylate, and poly vinyl pyrollidone.
The present invention however utilizes sulfonated fluorocarbon polymer and more preferably a sulfonated tetrafluoroethylene perfluoroether copolymer in thin film form to provide a moisture sensor capable of operating over a broad temperature range with exceptionally fast response time to changes in water content.
Sulfonated fluorocarbon polymers and copolymers suitable for this invention are sold commercially by the Dupont Company under the trademark "NAFION".
Sulfonated Fluorocarbon polymers have been used to advantage in hygrometric applications in the past, but not in the manner of the present invention. An early example of the use of sulfonated fluorocarbon polymers in tube form for measuring water content of a gas is disclosed in National Aeronautics and Space Administration Technical Brief, Vol. 3, No. 1, item #52 (June 1978) and in strip form in Vol. 8, No. 2, Item #139 (November 1984) for measuring trace amounts of water in space and the former of which is also disclosed in U.S. Pat. No. 4,514,278 and the latter of which is also disclosed in U.S. Pat.No. 4,514,278, the disclosures of both of which are incorporated herein by reference.
Other examples of the use of sulfonated fluorocarbon Polymers in tubing form in hygrometric applications are disclosed in U.S. Pat. No. 4,604,182; 4,612,019; 4,717,403; and 4,799,374, the disclosures of all of which are incorporated herein by reference.
Examples of the use of sulfonated fluorocarbons in the form of a membrane that is generally thinner than sheet and much thicker than film is disclosed in U.S. Pat. No. 4,549,134 in which the membrane has a thickness of 1/2 to 2.5 mil and in U.S. Pat. No. 4,693,953 where the membrane has a thickness of 178 .mu.m and in U.S. Pat. No. 4,528,078 where the membrane has a thickness of 170-180 .mu.m and in U.S. Pat. No. 4,681,855 where the fluorocarbon includes certain other pendent groups in combination with sulfonic pendent groups and the membrane has a thickness of 0.1 mm.
It has been discovered however that attractively accurate and fast response time to changes in moisture level can be achieved by use of extremely thin sulfonated fluorocarbon film having a thickness of less than about one micron and which characteristically cannot be handled like a membrane, sheet or tubing and must be coated on to an appropriate support substrate preferably by spin coating or drawing through a layer of the material floating on the surface of water by a process that is commonly known in the art as the "Langmuir/Blodgett" film technique.
Also discovered is a simple and economical way of making a moisture sensor utilizing a film of sulfonated fluorocarbon having a thickness of less than about one micron in addition to providing a system that is operative to process the electrical output signal of the moisture sensor in a highly effective manner and preferably deliver it to a display or the like for visual monitoring of moisture content, or electronics data acquisition.